For granulating plastics under water molten plastic is pressed through a perforated plate whereby one or more plastic strands are extruded. A rotor provided with several cutting edges is arranged coaxially to the perforated plate such that the plastic strands are pushed into the plane of the cutting edges, and thus they are chopped into small pieces, i.e. into granulate. The rotor is located in a casing flooded with cooling water whereby the cooling water is used for cooling the granulate as well as for transporting the granulate out of the granulating apparatus. Such a underwater granulating method as well as a corresponding apparatus is described, for example, in the publication EP 2 052 825 A2 (of ECON Maschinenbau and Steuerungstechnik GmbH), which is incorporated herein by reference. Instead of cooling water other liquids, for example glycol, may be considered.
The plastic granulate, which is generated using the above-mentioned underwater granulating methods, has to be isolated and dewatered or dried. In known methods the granulate is separated from a good portion of the water using a sieve and subsequently the still moist granulate is supplied to a centrifugal dryer in which the surface water adhering to the granulate is centrifuged off together with plastic powder which may be present.
Known centrifugal dryers usually include a cylindrical casing in which a cylindrical sieve is fixed. In the interior of the sieve a rotor is provided with paddles radially sticking out which, when the rotor is adequately driven, accelerate the still adhering water and thus centrifuging it through the sieve. The remaining moisture may be removed by vaporization whereby the vaporization process may be supported by a stream of air. Instead of the paddles radially sticking out baffle plates or the like may be provided.
In known centrifugal dryers the plastic granulate may be subject to considerable mechanical load due to the paddles or the baffle plates what may lead to an undesired generation of plastic powder. Further, such centrifugal dryers are not easy to maintain as they have to be entirely cleaned after each change of the granulate so as to avoid a contamination of the subsequent charge by potentially present residual plastic powder.
Accordingly, it would be desirable provide an apparatus for dewatering granulate which allows a gentle treatment of the material to be dewatered.